A portable impact tool, especially, a cordless impact tool which is driven by the electric energy accumulated in a battery is widely used. In the impact tool where a tip tool such as a drill or a driver is rotationally driven by a motor to perform a required work, the battery is used to drive a brushless DC motor, as disclosed in JP2008-278633A, for example. The brushless DC motor refers to a DC motor which has no brush (brush for rectification). The brushless DC motor employs a coil (winding) at a stator side and a permanent magnet at a rotor side and has a configuration that power driven by an inverter is sequentially energized to a predetermined coil to rotate the rotor. The brushless DC motor has a high efficiency, as compared to a motor with a brush and is capable of obtaining a high output using a rechargeable secondary battery. Further, since the brushless DC motor includes a circuit on which a switching element for rotationally driving the motor is mounted, it is easy to achieve an advanced rotation control of the motor by an electronic control.
The brushless DC motor includes a rotor having a permanent magnet and a stator having multiple-phase armature windings (stator windings) such as three-phase windings. The brushless DC motor is mounted together with a position detecting element configured by a plurality of Hall ICs which detect a position of the rotor by detecting a magnetic force of the permanent magnet of the rotor and an inverter circuit which drives the rotor by switching DC voltage supplied from a battery pack, etc., using semiconductor switching elements such as FET (Field Effect Transistor) or IGBT (Insulated Gate Bipolar Transistor) and changing energization to the stator winding of each phase. A plurality of position detecting elements correspond to the multiple-phase armature windings and energization timing of the armature winding of each phase is set on the basis of position detection results of the rotor by each of the position detecting elements.
FIG. 12 is a graph showing a relationship among a motor current, a duty ratio of PWM drive signal and a fastening torque in a conventional impact tool. Here, an operation for fastening a screw, etc., is performed in such a way that an operator pulls a trigger at time t0 to rotate the motor. At this time, the duty ratio 202 of the PWM drive signal is 100%. (3) of FIG. 12 represents a fastening torque value (N/m). The fastening torque value 203 is gradually increased with the lapse of time. Then, when a reaction force from a fastening member is equal to or greater than a predetermined torque value, the hammer is retracted relative to the anvil and therefore engagement relationship between the anvil and the hammer is released. As the engagement relationship is released, the hammer is rotated while moving forward and collides with the anvil at time t1 whereby a powerful fastening torque is generated against the anvil. At this time, the duty ratio of the PWM supplied to the inverter circuit for driving the motor is in a state of 100%, i.e., in a full power state, as indicated by the duty ratio 202 in. (2) of FIG. 12. The motor current in such a motor drive control is represented by the motor current 201 in (1) of FIG. 12. The motor current 201 is rapidly increased as indicated by an arrow 201a according to the retreat of the hammer and reaches a peak current (arrow 201b) just before the engagement state is released. Then, the motor current 201 is rapidly decreased when the engagement state is released. Then, striking is performed at an arrow 201c and the engagement state is obtained again, so that the motor current 201 begins to increase again.
Now, a relationship between movement of a striking part of the impact tool including the hammer and anvil and increase/decrease of the motor current will be described with reference to FIG. 13. A hammer 210 is moved forward and backward by the action of a cam mechanism provided in a spindle. The hammer is rotated in contact with an anvil while a reaction force from the anvil 220 is small. However, as the reaction force is increased, the hammer 210 begins to retreat to a motor side (upper side in FIG. 13) as indicated by an arrow 231 while compressing a spring along a spindle cam groove of the cam mechanism ((A) of FIG. 13). Then, when a convex portion of the hammer 210 rides over the anvil 220 by the retreat movement of the hammer 210 and therefore engagement between the hammer and the anvil is released, the hammer 210 is rapidly accelerated and moved forward (as indicated by an arrow 233) by the action of the cam mechanism and an elastic energy accumulated in the spring while being rotated (as indicated by an arrow 232) by a rotation force of the spindle ((B) of FIG. 13). Then, the convex portion of the hammer 210 collides with the anvil 220 and the hammer and the anvil are engaged with each other again, so that the hammer and the anvil begin to rotate integrally, as indicated by an arrow 234 ((C) of FIG. 13). At this time, a powerful rotational striking force is exerted to the anvil 22. A motor current 240 (unit: A) at this time is represented in a lower curve. The motor current 240 reaches a peak as indicated by an arrow 240a when the hammer is moved backward as indicated by the arrow 231 while compressing the spring along the spindle cam groove of the cam mechanism. Then, the engagement state between the hammer 210 and the anvil 220 is released, as shown in (B) of FIG. 13. At this time, the reaction force is not applied to the hammer 210 and therefore load becomes lighter. As a result, the motor current 240 is decreased, as indicated by an arrow 240b. Then, striking is performed in the vicinity where the motor current 240 is nearly decreased, as indicated by an arrow 240c. Here, the arrows 201b and 201c in FIG. 12 correspond to the portion of the arrows 240a to 240c in FIG. 13.
Explanation is made by referring to FIG. 12, again. In a case that a screw fastening member is a short screw, the striking may be performed at time t1 in FIG. 12 (i.e., at the time indicated by the arrow 201c) if a torque value suddenly exceed a setting torque value TN by the first striking, as indicated by an arrow 203a in (3) of FIG. 12. However, in the case of an electric tool that is not automatically stopped even when the torque value reaches the setting torque value, striking may be further performed several times before an operator releases a trigger. For example, in the example of (3) of FIG. 12, second striking is performed at time t2 and the motor current at this time is increased or decreased, as indicated by the arrows 201c to 201f. At this time, there is a possibility that screw threads are broken or a screw head is twisted and cut, in some cases.